The tryptophan specific tRNA ligase from B. Stearothermophilus has been crystallized in a form suitable for X-ray structure determination. Different crystal forms have been obtained for the native enzyme (Form I) and for enzyme plus tryptophan (Form II), enzyme plus ATP (Form III) and enzyme plus tryptophan plus ATP (Form IV). Unit cell spacings and a space group have been determined for Form II crystals. These data establish the basis for structure determination by X-ray diffraction methods. The objective is to solve the structure of Form II crystals using heavy atom isomorphous replacement methods. Subsequently it is proposed also to solve the structure of Forms I, III and IV. The long-range goal is to provide structural characterization of the enzyme at different stages of the aminoacyl adenylation reaction, and to apply this information to the problem of its catalytic mechanisms.